Broadband wireless

Broadband wireless communications could in principle use several physical media ranging from radio frequencies to light. The current focus is on the propagation of electromagnetic waves in frequency range form some hundreds of MHz to GHz. MMDS, MVDS and LMDS are among the newer systems that can be encompassed within the term 'Broadband Wireless'. MMDS (Multichannel Multi-point Distribution System) operate in the 2.5 - 3.6 GHz, while MVDS (Multi-point Video Distribution Service) and LMDS (Local Multi-point Distribution Service) operate at higher frequencies and within smaller cells, at 40 GHz and 28 GHz (upstream and downstream).

MVDS, MMDS and LMDS are identified as an alternative method for essentially local terrestrial television services. They can deliver many more channels than is possible with existing UHF terrestrial networks, with implicit requirement to use higher frequencies where spectrum is less congested. The rationale behind the developments was the desire to provide existing and future cable operators with a wireless radio based delivery mechanism. It was a means of augmenting normal cable coverage by dealing with pockets of franchised area usually considered not viable for cable deployment, an alternative to cable itself if economically viable and a means of providing a quick and efficient way to make available programming to customers.

MMDS is an adaptation of the system that has been in use in the US since the early 1960s to broadcast educational TV programmes to schools and colleges using frequencies around 2.5GHz.

The system has evolved to become a well established, proven, mature TV distribution technology which has taken place alongside cable, broadcast and satellite as a medium for transporting TV programmes direct to home and to cable head-ends.

Though MMDS came too early, it is well placed to exploit the emerging technology in the digital domain to extend the range of TV channels available to the viewers. Digital techniques will enable four or more TV channels, to be compressed into the bandwidth of a single analogue 8

MHz PAL TV channel, without noticeable degradation of picture quality. During 1994, test transmissions of digitally compressed high-definition TV were carried out over an Irish MMDS system under the European EU RACE programme. The compressed 34 Mbit/s HDTV signal was successfully transmitted over an MMDS system using COFDM modulation. The experience has established the viability of 2.5 GHz MMDS technology for multichannel TV delivery direct to home particularly in rural areas. With digital capability MMDS is another potential candidate for multimedia and data transmission.

5.3.8.1 Multichannel Multipoint Distribution Service (MMDS)

MMDS is more commonly known by the oxymoron "wireless cable". MMDS is a radio alternative to cable TV distribution. It may be particularly important in rural areas because it can be used to distribute up to 33 TV channels in some situations where there is neither local broadcasting nor sufficient customer density to support a regular cable system. In this context, it is primarily an alternative to satellite TV, with the additional advantage that MMDS can supply local channels.

MMDS is allowed to radiate up to 100 Watts per channel using an omni-directional antenna. The receivers, usually at a home, use a high-gain directional dish antenna (about 30" in diameter) and a down-converter (that usually converts the selected channel to ordinary VHF TV channel 2 or 3). Although the range of the service is generally limited to line-of-sight operation , line-of-sight can be 16-32 km if the terrain is favourable. The receiver requires only a simple frequency conversion, since the TV signal is already broadcast in the proper analogue NTSC, PAL or SECAM format, suitable for viewing on an unmodified TV set. However, the down-converter must also provide for the necessary decoding and access to prevent pirate viewing.

Typical costs of MMDS for the rural homeowner include a typical $30 monthly fee for antenna and down-converter rental and program subscription costs. It is claimed that an MMDS system operating in a rural environment can earn a profit with only 800 subscribers (about 10% of the total number of subscribers required for profitable operation of a normal cable system).

Local multipoint distribution service (LMDS) may provide a similar service as MMDS at some future time. LMDS uses frequency bands near 30 or 40 GHz. LMDS might be considered a higher frequency version of MMDS. LMDS is expected to provide general two-way services, including voice, video, and data in a large city environment, in addition to TV. This suggests that the role of MMDS might also be extended at some future time to include general telecommunications.

As in the cable TV industry, the MMDS industry is experimenting with digital compression techniques. For example, one system being tested puts 10 TV channels on a single 6-MHz bandwidth. Other systems provide 4:1 compression. The prospect of MMDS with 200-300 video channels completely changes the economics of providing MMDS. In addition, return path channels are mostly unused. Some operators have proposed using the MMDS channels (2150-2160 MHz) for return path channels. The use of return path channels could allow interactive video services such as home shopping and video on demand as well as general two-way telecommunications.

5.3.8.2 ISDN

The integrated services digital network (ISDN) is a set of common standards for combining separate telephone and data networks. Voice and data services are integrated in an ISDN network because they are processed in the same way as digital bits. This simplifies management since one network can support both types of information. It also eliminates some of the limitations of using the telephone network for data communications. Users can simultaneously talk and transfer large amounts of data over a single line. For subscribers with an ISDN connection, there is no need for a modem since digital data no longer has to be converted to analogue for transmission over the telephone network. This produces faster and more reliable connections, as well as higher data

transmission rates. ISDN is also more efficient than data transmission with a modem since control information is separated from the data being transferred, resulting in faster call set up and termination. The greater data transmission capability offered by ISDN should result in faster transfer and lower costs compared to the conventional telephone network.

ISDN is being standardized under the auspices of the ITU Telecommunication Standardization Sector (ITU-T), notably through the I-Series Recommendations. There are two types of ISDN implementations in use today that work over conventional copper wire telephone lines:

Basic rate access, which comprises two "B" or bearer channels of 64 kbit/s (56 kbit/s in North America and Japan) each allowing a total data transfer rate of 128 kbit/s or about 2.5 times faster than transfers using a telephone line and high speed modems. The B channel can also be used to carry a single voice channel or, with data compression, up to eight separate voice channels. There is also a "D" or data channel of 16 kbit/s, which carries control information or packet data.

The primary rate access consists of multiple B channels (30 in Europe, 23 in USA and Japan) providing service at 2.0 Mbit/s (1.544 Mbit/s).

There is no distinction between voice, text image or data on the B channel; they are all treated as strings of digital bits. When a call is made, the customer's equipment sends a signal on the D channel containing the information needed by the network to establish the call. Data or voice is then transmitted over the B channel. The customer's equipment sends a termination signal down the D channel to end the call.

5.3.8.2.1 Applications of ISDN

Extra telephone functionality: Information identifying the caller's telephone number (caller line identification, or CLI), for example, can be transmitted on the D channel. This information can be directed to an organization's computer database that can contain customer or supplier details, so that when the telephone is answered the respondent already has the details of the caller on-screen.

This function has particular applications in telemarketing, emergency services, and customer service help lines.

Enhanced audio: ISDN's high quality audio capability can be used for sound broadcast applications such as FM-fidelity stereo. It should be possible to use ISDN lines for distributing music, for instance, to relay live concerts to remote areas.

Connection of computer systems: This includes linking local area networks (LANs) and, via ISDN adapters, providing personal computers with rapid data transfer capability. ISDN connections have become popular in Europe recently for residential users and small businesses with dial-up access to the Internet. The 64 kbit/s connection is more efficient than using modems that often operate at 28.8 kbit/s due to telephone line quality. (Faster rates, up to 56 kbit/s, are available with currentmodem if the quality of the telephone line allows it.) Indeed, one of the reasons why ISDN subscribers have grown more rapidly is because this has coincided with a rapid increase in usage of the Web browsing capabilities on the Internet which makes heavy use of graphic images.

High-level application support: Video codecs (coders/decoders) for desktop conferencing and videophones can work at ISDN speeds, either with still images at 56/64 kbit/s or full motion at

1.5/2.0 Mbit/s. The relevant ITU-T Recommendations for audio-visual services form part of the H Series.

ISDN is an alternative for leased lines. ISDN should be less expensive than fixed leased line charges, though this is not always the case; ISDN is often priced as a premium rate service even though it uses the same copper wires as normal voice telephony. ISDN may be favoured by those organizations whose requirements for transmission capacity vary considerably according to how much capacity they need and when they need it. With primary rate ISDN, it is possible for the Post and Telecommunications Operator to provide bandwidth on demand by offering fractional speeds in response to short term demand.

ISDN supports the digital fax transmission Group 4 facsimile which is up to eight times faster than a Group 3 fax and has much lower error rates. Group 4 can also be used for transmission of colour fax.

5.3.8.2.2 ISDN developments

Existing narrowband ISDN technology has one big limitation. It assumes a basic channel of 64 kbit/s which cannot support services requiring large bandwidth such as full-motion video. Thus activity is underway to define a new set of standards for broadband-ISDN (B-ISDN) that would eliminate the fixed channel structure of narrowband ISDN. B-ISDN, for example, will provide bandwidths between 155 and 622 Mbit/s, 80-300 times greater capacity than the existing primary rate ISDN. This will support virtually any foreseeable commercial application. Customers could then be offered either a bandwidth-on-demand type service at fractional speeds, or a permanent bandwidth connection according to requirement and ability to pay.

The ITU-T work programme currently has more than 400 standards at different stages of drafting which relate in some way to ISDN. The goals of the standardization effort are to ensure global interconnectivity of national B-ISDN networks as well as standardized equipment specifications for all interfaces to the network, something, which has plagued narrowband ISDN. B-ISDN also aims is to achieve a tight integration with exchange and transport technologies to minimize limitations in the switching and transmission of broadband information. Asynchronous transfer mode (ATM) standards have been adopted as the preferred transmission mode for implementing B-ISDN. ATM is a cell-switching technique for the transfer of user information, where each cell has a fixed size of 53 bytes. This makes it particularly well-suited to transmission and switching of multimedia applications because of the ability to offer variable bit rates (bandwidth on demand) which is more difficult with packet-switching protocols, such as X.25 or frame relay.

5.3.8.2.3 Benefits of ISDN

There are a number of advantages for the users:

Higher quality of voice transmission is the most noticeable feature. Indeed, one US carrier uses the sound of a pin dropping in its advertisements to emphasize this feature. When voice, data or text is converted into a series of numerical values, it can be decoded by the called party's terminal with higher fidelity, a much lower error rate, and minimal interference.

Lower maintenance costs is a bonus mainly for the network operator but the subscriber also benefits since digitisation improves reliability. Progress towards digitisation in Morocco has been matched by a falling incidence of network faults.

Additional user functions become available to subscribers attached to digital networks. Many of these simply emulate functions available to users of business private branch exchanges, such as call forwarding or call waiting, but other functions actually utilize the data transmitted over the network. The most useful, and most controversial, is caller-ID, which allows the called party to see the number of the caller (unless the caller has specifically requested anonymity). Other functions include displaying cost of the call (for real-time billing and bill checking purposes), music on hold, teleconferencing, and allocating different dialing tones to different numbers addressable from the same telephone.

Use of data transmission facilities, both for receiving and making calls, is much easier on a digital network. Where ISDN service is available, data communications can be much cheaper because of the higher bandwidth, but even where ISDN is not used, a digital line will provide a much lower level of error transmission that an analogue line.